Electrochemical Sensor

ABSTRACT

An electrochemical sensor comprising a probe immersible in a measured medium and having at least two electrodes of a first electrically conductive material and at least one probe body of a second, electrically non-conductive material. The electrodes are at least partially embedded in the probe body and insulated from one another by the probe body, wherein the at least two electrodes are embodied of at least one conductive material and the probe body of at least one electrically insulating ceramic, wherein the electrodes are embodied of thin, measuring active layers of a conductive material and sit in an end face of the probe body of a ceramic material, and wherein the electrodes are electrically contacted via connection elements extending through the probe body.

The invention relates to an electrochemical sensor comprising a probeimmersible in a measured medium and having at least two electricallyconductive electrodes embedded in a ceramic probe body.

Electrochemical sensors are used in many fields, such as e.g. inclinical analysis or laboratory analysis, environmental protection, andprocess measurements technology. Electrochemical sensors work eitheraccording to a conductive, a potentiometric or an amperometric,measuring principle, such that the measured variable is ascertained inthe medium via the electrodes.

Known from the state of the art, e.g. from EP 990 894 B1, are conductiveconductivity sensors comprising at least two electrodes, which formeasuring are immersed in the measured medium. For determining theelectrolytic conductivity of the measured medium, the resistance orconductance of the electrode measuring path is determined in themeasured medium. In the case of known cell constant, the conductivity ofthe measured medium can be ascertained therefrom.

Shown in DE 10 2006 024 905 A1 is an electrode arrangement of aconductive conductivity sensor, in the case of which an inner and anouter electrode are isolated and insulated from one another by a shapedseal and a seal support body. The shaped seal serves to preventpenetration of measured medium into an annular gap between theelectrodes.

Such an electrode arrangement with additional seals is constructivelyrelatively complex and disturbance susceptible, so that medium canpenetrate into the gap between electrode and seal support body. Thestructural complexity is especially great in the case of conductivitysensors for application in foods technology or in the pharmaceuticalindustry. The sensors of process automation technology, which areapplied in the foods and/or pharmacy industries, must fulfill very highrequirements as regards hygiene. For example, the probes of suchsensors, to the extent that they come in contact with the measuredmedium, must not have difficultly accessible gaps, in order that acleaning and/or sterilizing of the total probe surface contacting themeasured medium is possible. Conventional seals or a shaped seal canaccording to DE 10 2006 024 905 A1, indeed, basically fulfill thispurpose. They lead, however, to a complex construction withcorresponding assembly complexity. Furthermore, with age and wear, theseseals can fail and then medium can get into the gap between electrodesand seal support body.

In general, the probe bodies of the probe of an electrochemical sensorare produced from a synthetic material by means of various manufacturingmethods, such as e.g. injection molding, impression molding, and hotstamping, into which the metal electrodes are installed. A greatdisadvantage of combining synthetic material, such as a plastic, and themetal electrodes are their different coefficients of thermal expansion.In the case of high loadings due to high surrounding pressures,respectively temperature fluctuations, gaps form between the differentmaterials of the probe body and the electrodes. This can lead to lack ofsealing of the sensor element, whereby medium can penetrate into thesensor interior. Furthermore, germs can get into these gaps, whereby thesensor cannot be qualified for hygienic uses. Another undesiredcharacteristic of synthetic materials is their poor long termdurability, since they age. Aging as a result of aggressive media orrepeated strong temperature changes increases the porosity of theapplied synthetic materials. In this way, it is possible that liquidmedium can diffuse through the synthetic material into the sensorinterior.

Shown in WO 2010/072483 A1 is a conductive conductivity sensor having aprobe immersible in a measured medium. The probe comprises at least twoelectrodes of a first electrically conductive material and at least oneprobe body of a second electrically non-conductive material. Theelectrodes are embedded in the probe body and insulated from one anotherby the probe body. Thus, the electrodes and the probe body are embodiedas a sintered, composite piece. To accomplish this, the probe bodyand/or the electrodes are produced by means of a multicomponentinjection molding process.

It is, consequently, an object of the invention to provide anelectrochemical sensor having a probe immersible in a measured medium,which overcomes the disadvantages of the state of the art as regardssealing between the electrodes and the probe body, whereby theavailability of the sensor is greatly increased, while manufacturingcosts are reduced.

This object is achieved by an electrochemical sensor comprising a probeimmersible in a measured medium and having at least two electrodes of afirst electrically conductive material and at least one probe body of asecond, electrically non-conductive material, wherein the electrodes areat least partially embedded in the probe body and insulated from oneanother by the probe body, wherein the at least two electrodes areembodied of at least one conductive material and the probe body of atleast one electrically insulating ceramic, wherein the electrodes areembodied of thin, measuring active layers of a conductive material andsit in an end face of the probe body of a ceramic material, and theelectrodes are electrically contacted via connection elements extendingthrough the probe body.

The embodiment of the electrodes as thin material layers with connectionelements extending through the probe body and their embedding in aceramic probe body achieves a gap-free material transition and therewithalso a gap-free sealing between the electrodes at least partiallyembedded in the probe body and the probe body.

In an advantageous embodiment, the measuring active layer of theconductive material of the electrodes has a coating thickness d of, forexample, 10 μm-3 mm. This measuring active layer of the electrodes sitsgap-freely in the ceramic material of the probe body, so that the endfaces of the electrodes and the probe body form a plane. The coatingthickness of the electrodes is, in such case, preferably in the range,10 μm to 200 μm, whereby through minimal use of noble metals, such ase.g. platinum, titanium and stainless steel, also costs can be saved.These thin layers of concentrically arranged ring-electrodes areelectrically contacted via corresponding connection elements.

In an additional embodiment, the conductive material comprises anelectrically conductive ceramic, electrically conductive enamel or ametal, especially platinum, titanium or stainless steel.

In an advantageous embodiment, the ceramic material comprises at least azirconium oxide (ZrO₂) ceramic, an aluminum oxide (Al₂O₃) ceramic, achromium oxide (Cr₂O₃) ceramic, a titanium dioxide (TiO₂) ceramic,and/or a tialite (Al₂TiO₅) ceramic.

In an especially suitable further development, the electrodes compriseplatinum and the probe body comprises a zirconium oxide ceramicstabilized by means of magnesium. The platinum of the electrodes and thezirconium oxide ceramic partially stabilized or stabilized withmagnesium have approximately the same thermal coefficients of expansion,for example, zirconium oxide stabilized with magnesium ZrO₂MgO at9.3×10⁻⁶ K⁻¹ and platinum Pt at 8.8×10⁻⁶ K⁻¹. For equalizing the thermalcoefficients of expansion of the ceramic material of the probe body andthe coefficients of expansion of the metal material of the electrodes,stabilizing materials, such as, for example, magnesium, iridium and/oraluminum are added into the ceramic material of the probe body. Theseadditions of stabilizing materials stabilize or at least partiallystabilize the ceramic material, so that the thermal coefficients ofexpansion of the probe body and the electrodes are approximately equaland also other properties of the material of the probe body, such as,for example, greater chemical durability, better fracture behavior,etc., result. For this reason, the solid composite of electrodes andprobe body remains stable over a large temperature range of, forinstance, −30° C. up to 300° C. This solid composite of the metalmaterial of the electrodes and the ceramic material of the probe bodyresults at least partially from intermolecular interactions or chemicalbonds between regions of the metal material of the electrodes andregions of the ceramic material of the probe body. In this way, thereresults a high quality, material bonded connection between theelectrodes and the probe body, which provides a gap-free seal. Becauseof the almost equal coefficients of expansion of the two materials,these bonding forces are also not overcome by otherwise arisingmechanical stresses upon temperature changes, so that gap formationbetween the electrodes and the probe body is prevented.

In an additional advantageous embodiment, the probe body is connectedwith a process connection. By connecting the probe body to the processconnection, an option is provided for applying the probe in processmeasurements technology directly and sealingly on the process container.

In an alternative embodiment, the process connection is embodied asone-piece with the probe body of the same electrically insulatingceramic. Ideally, the process connection is a component of the basicbody of the probe, i.e. embodied as one-piece with the probe body,respectively embodied as a single molded part. This has the advantagethat also the process connection is gap-free, due to the one-pieceembodiment, so that the total conductivity sensor has no gaps. In afurther development, for improving mechanical stability, respectivelyfor securement of the sensor, metal parts or parts of synthetic materialcan be provided on the side of the process connection facing away fromthe process.

In a special further development, the process connection is connected ata joint mechanically and sealingly with the probe body by means of ajoining means. Applied as joining means is an adhesive, which connectsthe metal process connection with the ceramic probe body and seals thejoint, respectively the joining gap, gap-freely.

In an additional embodiment, the electrochemical sensor is embodied as aconductive conductivity sensor. Conductive conductivity sensors areapplied in varied applications for measuring conductivity of a medium.The most known conductive conductivity sensors are the so-called two, orfour, electrode sensors. Two electrode sensors have two electrodes inmeasurement operation immersed in the medium and supplied with analternating voltage. A measuring electronics connected to the twoelectrodes measures an electrical impedance of the conductivitymeasurement cell, from which then, based on a cell constant determinedearlier from the geometry and character of the measuring cell, aspecific resistance, respectively a specific conductance, of the mediumlocated in the measuring cell is ascertained. Four electrode sensorshave four electrodes immersed in the medium during measurementoperation, of which two are operated as so called electrical currentelectrodes and two as so called voltage electrodes. Applied between thetwo electrical current electrodes in measurement operation is analternating voltage, so that an alternating electrical current flowsthrough the medium. This electrical current creates between the voltageelectrodes a potential difference, which is determined by a preferablycurrentless measurement. Also here, a measuring electronics connected tothe electrical current, and voltage, electrodes determines from theintroduced alternating electrical current and the measured potentialdifference the impedance of the conductivity measurement cell, fromwhich then, based on a cell constant determined earlier from thegeometry and character of the measuring cell, a specific resistance,respectively a specific conductance, of the medium located in themeasuring cell is determined.

The object is achieved, furthermore, by a method for manufacturing aconductive conductivity sensor in one of the above describedembodiments, comprising steps as follows:

-   -   producing in a first step a green body of the probe body from        the electrically insulating ceramic,    -   in a second step, pressing the electrodes with their connection        elements into the green body or introducing the electrodes with        their connection elements into corresponding cavities in the        green body,    -   sintering in a third step the green body with the introduced,        respectively pressed in, electrodes and connection elements.

For manufacturing the ceramic green body, all known methods can be used.Examples include:

-   -   ceramic slip casting    -   injection molding or temperature-inverse injection molding    -   sheet casting    -   extrusion    -   assembly of plates    -   chip removing methods, e.g. in a lathe or milling machine    -   pressing (uniaxial pressing, cold isostatic pressing, hot        isostatic pressing)

With this method, it is possible to produce the desired solid compositeof the electrodes of metal and the ceramic probe body, at least in aportion of a material transition, especially by intermolecularinteractions or chemical bonds, such as earlier described.

In a further embodiment of this method, the process connection ismechanically stably and sealingly connected with the probe body at ajoint by means of a joining means, especially by means of an adhesiveconnection, and the region of the joint after the joining togetherand/or the end face of the probe body with the therein gap-freelyembedded electrodes are/is ground or machined. Thus, the probe end face7 and the joint 8 of the adhesive connection between the probe body 3and the process connection 6 are ground, respectively machined, so thata planar, gap-free surface is obtained for the end face 7 and the joint8.

The invention will now be explained in greater detail based on theexamples of embodiments shown in the drawing, the figures of which showas follows:

FIG. 1 a probe of an electrochemical sensor, especially a conductivitysensor, according to a first embodiment of the invention,

FIG. 2 a probe of an electrochemical sensor, especially a conductivitysensor, according to a second embodiment of the invention,

FIG. 3 a probe of an electrochemical sensor, especially a conductivitysensor, according to the second embodiment of the invention of FIG. 2with a diameter expansion of the process connection at the joint.

FIG. 1 shows a probe 1 of the invention for an electrochemical sensor,especially a conductivity sensor, with a probe body 3 of an electricallynon-conductive, ceramic material and, according to the invention,therein embedded electrodes 5 of a thin, electrically conductivematerial. The coating thickness of the material of the electrodes 5 ofthe invention, which are provided in FIG. 1 as concentric rings,respectively sleeves, sintered into the probe body 3, lies in a range of10 micrometer to 3 millimeter, whereby material for the manufacture ofthe probe and, thus, costs, are saved. The end faces of the electrodes 5lie freely exposed on the end face 7 of the probe body 3 and in the caseof a measuring of conductivity they are in contact with the measuredmedium. FIG. 1 shows a perspective view of the probe 1 and shows,concentrically arranged around the rotational symmetry axis Z, the ringelements of the electrodes 5, which in the case of a measuring ofconductivity are immersed in the measured medium. Electrodes 5 areembodied as ring elements coaxially arranged around the sharedrotational symmetry axis Z and are embedded in the sensor body 3insulated from one another. Probe 1 is embodied as a measuring probe ofa 4-electrode sensor. In the case of this type of sensor, in measurementoperation, an alternating voltage is applied to the two electrodes 5 ofthe electrical current electrodes and the potential differencedetermined on the other two, remaining electrodes of the voltageelectrodes. Using a measurement transmitter (not explicitly shown)connected with the electrodes 5, the impedance of the conductivitymeasurement cell formed by the probe 3 immersed in the measured mediumis ascertained. Taking into consideration the cell constants, thespecific resistance, respectively the specific conductivity, of themeasured medium can be ascertained therefrom. The ascertained measuredvalues can either be displayed by the measurement transmitter or outputto a superordinated control system. A part the functions of themeasurement transmitter can be executed by a measuring electronicsaccommodated in a separate housing outside of the measurementtransmitter. This measuring electronics can, at least in part, beaccommodated, for example, in a plug head connected with the probe 1,which plug head is available from the applicant under the mark,MEMOSENS®.

The electrodes 5 are platinum and the probe body 3 a zirconium oxideceramic stabilized, respectively partially stabilized, by means ofmagnesium. The platinum of the electrodes 5 and the zirconium oxideceramic of the probe body 3 stabilized with magnesium possessapproximately the same thermal coefficients of expansion, for example,with magnesium stabilized zirconium oxide ZrO₂MgO being at 9.3×10⁻⁶ K⁻¹(per degree Kelvin) and platinum Pt at 8.8×10⁻⁶ K⁻¹. There are, however,other such material combinations for the electrodes 5 and the probe body3, whose thermal coefficients of expansion differ only little from oneanother, i.e. preferably deviating from one another by only 1×10−6 to2×10⁻⁶ K⁻¹. Thus, for example, in the case of platinum as material forthe electrodes 5, which has a thermal coefficient of expansion of8.9×10⁻⁶ K⁻¹, such can be combined with an aluminum oxide ceramic with acoefficient of expansion of 6 to 8×10⁻⁶ K⁻¹. In the case of titaniumwith a coefficient of expansion of 10.8×10⁻⁶ K⁻¹ as electrode material,such can be used with, for example, zirconium oxide ceramic with acoefficient of expansion of 10 to 12×10⁻⁶ K⁻¹ as material for the probebody 3. A zirconium oxide ceramic for the probe body 3 is likewisesuitable for combination with stainless steel as material for electrodes5, since stainless steel has a thermal coefficient of expansion of about13×10⁻⁶ K⁻¹.

Through the situating of metal in a ceramic shape, e.g. by sintering,the metal of the electrodes 5 is surrounded in a shape-interlockingmanner by the ceramic material of the probe body 3 and there arisesalso, such as earlier described, a material bonding between the twomaterials. For situating the electrodes 5 in the probe body 3, theelectrodes are seated in cavities provided in the probe body 3 orslightly pressed into the green body of the probe body 3. Afterinsertion of the electrodes 5 into the ceramic green body of the probebody 3, the assembly is sintered by means of a predetermined temperatureregimen.

The electrodes 5 can also be produced by deposition of the conductivematerial into corresponding cavities in the probe body 3. The followingmethods can be used for the deposition:

-   -   vapor deposition of metals    -   sputtering of metals    -   screen printing with metal pastes

In supplementation, also the probe body 3 can be produced by thefollowing deposition methods from a gas phase or liquid phase:

-   -   Chemical vapor deposition (CVD)—In such case, a plurality of        gases react with one another at a certain pressure and high        temperatures and deposit a ceramic material.    -   Physical vapor deposition (PVD)    -   Chemical vapor infiltration (CVI)

Since the coefficients of expansion of ceramics, such as e.g. zirconiumoxide and metal, preferably platinum, are almost identical, gapformation can be minimized. Furthermore, such a ceramic is suited due toits poor electrical conductivity as a support material for electricalmeasurements between the electrodes 5. Furthermore, ceramics are verysuitable support material due to their very good chemical durability.Ceramics have the property that they age very much slower than syntheticmaterials, which leads to a very much longer service life of the sensor.The surface roughness of the end faces 7 of the electrodes and/or of theprobe body 3, as well as the joint 8 between probe body 3 and processconnection 6, is further reduced by polishing processes after themanufacture, so that possibly arising gaps and openings on the outersurface of the ceramic probe body 3 are removed and, thus, the highhygienic requirements of the probe 1 can be durably fulfilled.

Used as electrically conductive material can also be an electricallyconductive ceramic, respectively enamel, which is cast, injected,respectively introduced into the corresponding cavities in the greenbody of the probe body 3 and after introduction sintered together withthe green body of the probe body 3. This embodiment has the advantagethat the used materials and, thus, the coefficients of expansion arevery similar.

Embedded in the probe body 3 and in the process connection 6 are theelectrodes 5 of the probe 1, which are electrically contacted viaconnection elements 2, respectively connection lines. Provided for this,for example, in a region of the sensor body 3 and of the processconnection 6 facing away from the process are connection elements 2, viawhich the electrodes 5 can be connected with a control or measuringelectronics.

Used for measuring the current temperature of the medium can be,furthermore, a temperature sensor 4. Temperature sensor is inserted viaa cavity provided in the probe body 3 facing away from the medium,respectively held in place with a thermally conductive adhesive. Bymeans of this temperature sensor 4, the current temperature of themedium on the electrodes 5 can be ascertained and, thus, a thermalcorrection of the conductivity measurement performed.

Probe 1 shown in FIG. 2 forms the measuring probe of a so-called4-electrode sensor immersible in a measured medium. Two electrodes 5,especially two electrodes 5 directly adjoining one another, are operatedas so called electrical current electrodes. The two remaining electrodes5 are operated as voltage electrodes. Applied between the two electricalcurrent electrodes in measurement operation is an alternating voltage,in order to introduce an alternating electrical current into themeasured medium. Measured between the voltage electrodes, especiallyusing a currentless measuring, is the resulting potential difference.Using the introduced alternating electrical current and the measuredpotential difference, the impedance of the conductivity measurement cellformed through immersion of the probe 1 in a measured medium iscalculated, and from the impedance while taking into consideration thecell constant, the specific resistance, respectively the conductivity,of the measured medium can be ascertained. Serving for control of theintroduced alternating current for measuring the potential difference ofthe voltage electrodes and converting the measured values into aresistance, respectively conductance or a specific resistance,respectively specific conductivity of the measured medium is ameasurement transmitter (not explicitly shown) connected with the probe1. The measuring electronics can be a component of the measurementtransmitter or at least partially accommodated in a separate module, forexample, in a plug head connected with the probe 1. The ascertainedmeasured values can either be displayed by the measurement transmitteror output to a superordinated control system.

As described in WO 2010/072483 A1, the probe 1 can also be produced in asingle method step by means of a two component, injection moldingmethod. In the case of this method, preferably an injection moldingmachine with two injection units is used. In the case of application ofone injection unit for the electrode material and an additionalinjection unit for the material of the sensor body, the two injectionunits are preferably controlled independently of one another, since, inthis way, a larger variety of electrode geometries can be produced. Twocomponent injection molding is a technology established especially forthe manufacture of components of different synthetic materials. Theinjection molding of metals or ceramics, for example, by means of metalpowder injection molding (MIM—Metal Injection Molding) or ceramic powerinjection molding (CIM—Ceramic Injection Molding), is a known andestablished manufacturing method for technically demanding and complexmolded parts. Also, multicomponent injection molding of metals and/orceramics as individual components is, in principle, known, however,previously not usual in the manufacturing of composites of metal andceramic.

In FIGS. 2 and 3 of the probe 1, the probe body 3 is joined with aprocess connection 6. For this, the probe body 3 is connectedmechanically stably and sealingly with the process connection 6, forexample, by means of an adhesive. The joint 8 between the sensor body 3and the process connection can be further worked by means of machining,grinding, and/or polishing. In this way, also adhesive residues areremoved. The diameter of the process connection 6 and of the probe body3 is enlarged at least in this region of the subsequent working of thejoint 8. In order that the adhesive gap be as small as possible, thus,as hygienic as possible, the lower end of the process connection 6 aswell as the ceramic sensor body 3 are provided with a diameter largerthan desired in the target application. Through subsequent grinding ormachining of the joint 8 of the connection between sensor body 3 andprocess connection 6, a region with very much smaller surface roughnessis produced. Thus, also highest hygienic requirements can be fulfilled.

The measuring active layer of the conductive material of the electrodes5 is embodied in a coating thickness d of, for example, 10 μm-3 mm andso seated in the probe body 3 that its end faces 7 lie in a plane A. Thethickness d, respectively height, of the electrodes 5 as well as theirdiameter D amounts in the embodiment of a four electrode measuring probe1 of FIG. 2 or FIG. 3 to preferably 1 to 2 millimeter.

LIST OF REFERENCE CHARACTERS

1. probe

2. connection elements

3. probe body

4. temperature sensor

5. electrodes

6. process connection

7. end face

8. joint

9. enlarged diameter

A plane of the end faces

Z axis of the concentric arrangement

d coating thickness

D diameter

1-12. (canceled)
 13. An electrochemical sensor comprising: a probeimmersible in a measured medium and having at least two electrodes of afirst electrically conductive material and at least one probe body of asecond, electrically non-conductive material, wherein: said electrodesare at least partially embedded in said probe body and insulated fromone another by said probe body; said at least two electrodes areembodied of at least one conductive material and said probe body of atleast one electrically insulating ceramic; said electrodes are embodiedof thin, measuring active layers of a conductive material and sit in anend face of said probe body of a ceramic material; and said electrodesare electrically contacted via connection elements extending throughsaid probe body.
 14. The electrochemical sensor as claimed in claim 13,wherein: said measuring active layer of the conductive material of saidelectrodes has a coating thickness d of 10 μm-3 mm; and said measuringactive layer sits gap-freely in the ceramic material of said probe body,so that the end faces of said electrodes and said probe body form aplane (A).
 15. The electrochemical sensor as claimed in claim 14,wherein: the conductive material comprises one of an electricallyconductive ceramic, an electrically conductive enamel and a metal,especially platinum, titanium or stainless steel.
 16. Theelectrochemical sensor as claimed in claim 13, wherein: the ceramicmaterial comprises at least a zirconium oxide (ZrO₂) ceramic, analuminum oxide (Al₂O₃) ceramic, a chromium oxide (Cr₂O₃) ceramic, atitanium dioxide(TiO₂) ceramic, and/or a tialite (Al₂TiO₅) ceramic. 17.The electrochemical sensor as claimed in claim 13, wherein: saidelectrodes comprise platinum; and said probe body comprises a zirconiumoxide ceramic stabilized by means of magnesium.
 18. The electrochemicalsensor as claimed in claim 13, wherein: said probe body is connectedwith a process connection.
 19. The electrochemical sensor as claimed inclaim 13, wherein: said process connection is embodied as one-piece withsaid probe body of the same electrically insulating ceramic.
 20. Theelectrochemical sensor as claimed in claim 13, wherein: said processconnection is connected at a joint mechanically and sealingly with saidprobe body by means of a joining means.
 21. The electrochemical sensoras claimed in claim 13, wherein: said electrodes are ring-shaped andarranged concentrically about a shared axis.
 22. The electrochemicalsensor as claimed in claim 13, wherein: the electrochemical sensor isembodied as a conductive conductivity sensor.
 23. A method formanufacturing an electrochemical sensor, comprising the steps of:producing in a first step a green body of a probe body from theelectrically insulating ceramic; in a second step, pressing electrodeswith their connection elements into the green body or introducing theelectrodes with their connection elements into corresponding cavities inthe green body; and sintering in a third step the green body with theintroduced, respectively pressed in, electrodes and connection elements.24. The method for manufacturing an electrochemical sensor as claimed inclaim 23, wherein: a process connection is mechanically stably andsealingly connected with the probe body at a joint by means of a joiningmeans, especially by means of an adhesive connection; and the region ofthe joint after the joining together and/or the end face of the probebody with the therein gap-freely embedded electrodes are/is processedsuch that material is removed.